In mobile communication devices, RF filters are used in order that in a frequency-dependent fashion selectively desired signals which propagate in a signal line are allowed to pass as much as possible without being changed and in order that undesired signals are prevented from propagating in the signal path. In a duplexer, for example, RF filters serve to allow a transmission signal to pass from the transmission signal path to the antenna and a reception signal to pass from the antenna to the reception path. RF filters which are only transmissive for signals and are interconnected to form a duplexer enable a mobile communication device to transmit and receive RF signals simultaneously in the different frequency bands. For this purpose, in the duplexer, it is then respectively the case that a transmission filter is interconnected between a transmission signal path and an antenna and a reception filter is interconnected between an antenna and a reception path.
Major challenges for the filters in the duplexer are, firstly, to obtain an insertion loss of the desired signals that is as low as possible and, secondly, to suppress undesired signals to the greatest possible extent in the respective signal path, i.e., to obtain good stop band suppression. At the same time, the intention is to ensure good isolation between the transmission signal path and the reception signal path. In this case, one measure of the performance of a duplexer is the scattering matrix S. In this case, the absolute value of the matrix element S12, |S12|, indicates the frequency-dependent attenuation (insertion loss) of RF signals which are conducted from the transmission signal path to the antenna. The absolute value of the matrix element S23, |S23|, denotes the frequency-dependent attenuation (insertion loss) of RF signals which are conducted from the antenna into the reception path. The absolute value of the matrix element S13, |S13|, denotes the frequency-dependent attenuation (isolation) between transmission signal path and reception signal path. That is to say that S13 indicates that frequency-dependent power which, generally undesirably, is coupled from the transmission signal path directly into the reception signal path. A high isolation, i.e., a high attenuation of these signals, is desirable.
The trend toward ever more extensive miniaturization of RF circuits and, in particular, of mobile communication devices presents the developers of these devices with the challenge of obtaining a sufficient ratio (selection) of low insertion loss in the passband and high attenuation in the stop band, despite reduction in the size of the components. At the same time, a sufficient isolation between a transmission path and a reception path in the duplexer is also intended to be made available.
U.S. Pat. No. 7,116,187 B2 discloses a ladder-type-like filter circuit for RF signals. Ladder-type filter circuits consist of cascaded basic elements. The individual basic elements each comprise a series resonator and a parallel resonator. In this case, series and parallel resonators of adjacent basic elements can be combined. In this case, the series resonators of all the basic elements are connected in series in a signal path, while the parallel resonators interconnect that section of the signal path which is associated with the corresponding series resonator of the basic element with ground. U.S. Pat. No. 7,116,187 proposes implementing the ground links of the parallel resonators via a jointly utilized inductive element. What is disadvantageous about such a circuit design is that an additional inductive element has to be provided and accommodated in structural space that already has barely sufficient dimensions anyway.